1. Field of the Invention
This invention relates generally to methods and apparatus for transforming energy from a heat source into usable form using a working fluid that is expanded and regenerated. This invention further relates to a method and apparatus for improving the heat utilization efficiency of a thermodynamic cycle.
2. Brief Description of the Background Art
In the Rankine cycle, the working fluid such as water, ammonia or a freon is evaporated in an evaporator utilizing an available heat source. The evaporated gaseous working fluid is expanded across a turbine to transform its energy into usable form. The spent gaseous working fluid is then condensed in a condenser using an available cooling medium. The pressure of the condensed working medium is increased by pumping, followed by evaporation, and so on to continue the cycle.
The basic Kalina cycle, described in U.S. Pat. No. 4,346,561, utilizes a binary or multi-component working fluid. This cycle operates generally on the principle that a binary working fluid is pumped as a liquid to a high working pressure and is heated to partially vaporize the working fluid. The fluid is then flashed to separate high and low boiling working fluids and the low boiling component is expanded through a turbine to drive the turbine, while the high boiling component has heat recovered for use in heating the binary working fluid prior to evaporation. The high boiling component is then mixed with the spent low boiling working fluid to absorb the spent working fluid in a condenser in the presence of a cooling medium.
A theoretical comparison of the conventional Rankine cycle and the Kalina cycle demonstrates the improved efficiency of the new cycle over the Rankine cycle when an available, relatively low temperature heat source such as ocean water, geothermal energy or the like is employed.
In applicant's further invention, referred to as the Exergy cycle, the subject of U.S. patent application Ser. No. 405,942, filed Aug. 6, 1982, now U.S. Pat. No. 4,489,563 relatively lower temperature avilable heat is utilized to effect partial distillation of at least a portion of a multicomponent working fluid stream at an intermediate pressure to generate working fluid fractions of differing compositions. The fractions are used to produce at least one main rich solution which is relatively enriched with respect to the lower boiling component, and to produce at least one lean solution which is relatively impoverished with respect to the lower boiling component. The pressure of the main rich solution is increased; thereafter, it is evaporated to produce a charged gaseous main working fluid. The main working fluid is expanded to a low pressure level to convert energy to usable form. The spent low pressure level working fluid is condensed in a main absorption stage by dissolving with cooling in the lean solution to regenerate an initial working fluid for reuse.
The inventor of the present invention has appreciated that it would be highly desirable to enable the efficient use of a very low pressure and temperature fluid at the turbine outlet, in the Exergy cycle. Regardless of the temperature of the cooling water in the condenser, the higher the pressure of condensation in the Exergy cycle, the higher is the concentration of the lower boiling component in the basic solution. However, the higher the pressure of condensation, the higher the pressure at the turbine outlet and the higher the concentration of the lower boiling component at the turbine outlet. This higher concentration basic solution requires for distillation, heat of a lower temperature. Thus, by reducing the pressure, and consequently the temperature at the turbine outlet, the concentration of the lower boiling component of the basic solution may be lowered and a higher temperature may be required at the turbine outlet to provide for distillation.
This contradiction might be addressed by balancing the pressure at the turbine outlet with the cooling water temperature. However, to achieve the maximum power output, the turbine outlet pressure must be as low as possible. When the turbine outlet pressure and temperature are reduced, as described above, the concentration of the lower boiling component of the basic solution decreases. This results in a cycle requiring exactly the opposite action to increase the turbine outlet pressure and temperature. The situation worsens with higher available cooling water temperature.
The inventor of the present invention has also appreciated the desirability of controlling the outlet temperature of the fluid exiting the turbine in the Exergy cycle. The efficiency of a thermodynamic cycle such as the Exergy cycle may be improved by heating the fluid in the boiler to the highest possible temperature with the available heat source. However, it is still desirable that the fluid exiting from the turbine be at a temperature and pressure close to that of a saturated vapor. To the extent that the exiting vapor is superheated, exergy is wasted.
It is particularly desirable in the Exergy cycle to obtain only slightly superheated vapor or saturated vapor from the turbine while inputting fluid at the highest possible temperature to the turbine. This is because in the Exergy cycle the output from the turbine is not simply condensed, but instead is used for distillation. The superheating of the fluid outletted from the turbine may cause unnecessary exergy losses in the cycle as a whole. For example, since the spent fluid from the turbine may be used to pre-heat the condensed fluid in a heat exchanger prior to regeneration, as described in the aforementioned patent application, an inefficiently high temperature difference may exist in the heat exchanger.
If one attempts to overcome this problem by further fluid expansion in the turbine, one obtains a lower temperature at the turbine outlet but a lower pressure as well. This lower pressure fluid is more troublesome to distill because more heat is required and this lower pressure fluid requires a larger quantity of lean solution to absorb it. Thus, this approach to the solution of the problem of exergy losses arising from the high temperature of the fluid exiting the turbine is not desirable.